The present invention relates to a method of producing two types of hybrid maize in a field such that they may be harvested independently.
Maize is a commodity grain in the United States used for a variety of applications including as a human food source, an animal feed, and as a source of carbohydrate, oil, protein, and fiber. However, there exists at present a growing market for maize with special end-use properties which are not met by maize grain of standard composition. Most commonly, specialty maize is differentiated from “normal” field maize by altered endosperm properties, such as an overall change in the degree of starch branching (waxy maize and amylose extender), increased accumulation of sugars or water-soluble polysaccharides (sugary, shrunken, and supersweet maize) or alterations in the degree of endosperm hardness (food grade maize or popcorn).
The need for such specialty maize arises, inter alia, from the need for starches with specific functionality or properties for use in a variety of industrial applications including paper additives such as sizings, adhesives, food components such as thickeners, industrial and medical absorbants, pharmaceutical excipients, cosmetics, and delivery systems such as emulsifiers.
Virtually all commercial maize produced in the United States, Canada, and Europe, and much of the maize produced in South America, is produced from hybrid seed. The production of maize hybrids requires the development of elite maize inbred lines that demonstrate good general and specific combining ability in order that they produce agronomically superior hybrids. Among the traits that plant breeders select for in producing hybrids are high yield, fast grain drydown, and resistance to specific plant diseases and insects.
Once elite inbreds have been developed, they may be used in several ways to produce commercial hybrid seed. The majority of hybrid seed produced in the United States is of the single cross type. Two inbred lines are intermated to give rise to what is termed an F1 single cross hybrid (A×B). In some instances, the female parent in the cross is itself an F1 hybrid, so that a three-way cross hybrid is produced with the genotype of (A×B)×C. More rarely, a four-way cross hybrid is produced, with both male and female parents as F1 hybrids, resulting in a genotype of (A×B)×(C×D). In all cases, the resulting kernels from this intermating are sold as seed to commercial growers who ultimately harvest F2 grain from the crop for on farm use or commercial sale.
In addition to possessing the proper combination of genetic factors to produce elite hybrids, the inbreds themselves must be reasonably vigorous to support the demands of modern seed production. This can be illustrated by a description of how single cross hybrids are produced commercially. To control the direction of pollination and assure the harvest of predominantly hybrid seed, seed production fields are typically designed so that 4 rows of inbred maize plants serving as females (male sterile) alternate with 1 row of inbred maize plants serving as males (male fertile). The female plants are rendered male sterile such that ovules borne on these female plants are then fertilized by pollen produced by the male plants, and the resulting hybrid seed borne on the female plants is harvested, cleaned, sized, and treated prior to sale to commercial growers. To produce this hybrid seed economically the male inbred plants need to reliably shed sufficient pollen to fertilize the female plants over a variety of climatic conditions. The hybrid seed borne on the female inbred plants need to be of high quality to allow good germination and early plant vigor in the commercial grower's field, and the female plants themselves need to stand and retain ears until the time of harvest.
There are numerous breeding schemes used to produce inbred lines of maize including the pedigree system of breeding, backcross conversion, and recurrent selection. All of these schemes are labor and capital intensive, each requiring many years of effort to allow for both recombination of genetic information and selection to eventually produce elite inbred lines. The rapidity with which satisfactory inbred lines can be developed is determined to a large degree by the nature and number of traits that the lines must possess as dictated by the plant breeder. The addition of novel or unusual traits, especially if controlled by many genes as in the case of oil content, would significantly increase the time and effort required to produce the desired lines.
More recently, alternative methods have been developed for producing maize hybrids with unique functionality. For example, U.S. Pat. Nos. 5,704,160 and 5,706,603 introduce methods of producing maize with enhanced traits such as high oil. The entire field of maize is then harvested together, resulting in a single intermediate trait maize. In the alternative, only one type of maize is harvested and the other is allowed to go to waste.
Development of corn hybrids with unique starch functionality is similar to commercial corn breeding. Altered starch functionality can be achieved by incorporating recessive forms of genes involved in the starch biosynthetic pathway. These genes are commonly referred to as kernel mutant genes. Kernel mutant genes can often cause reduced starch accumulation and difficulties in the wet milling operations designed to isolate the starch from other fractions of the corn kernel.
Surprisingly, it has now been discovered that by planting two different maize hybrids in alternating blocks of rows of sufficient width to efficiently harvest separately, maximum functionality of the maize kernel trait may be achieved. Further, by using one hybrid which is homozygous for two recessive kernel mutant genes and a second hybrid which is homozygous for only one of the two recessive genes, the desired functionality of the starch may be realized without the usual drawbacks of double recessive hybrids.